Standard signaling systems, such as signaling systems based on light-emitting diodes (LED's), are very hard to read/see when illuminated by sunlight under a low-angled sun (such systems are typically oriented vertically when used in traffic). All inexpensive standard plastic LED's, such as 3, 5, 8, 10 mm, round or oval shaped LED's, comprise an embedded plastic lens and internal reflector which are arranged for beaming the emitted light straight to the viewer (under various angles of +/−1o 180 degrees horizontal or vertical depending on the model). But these standard plastic LED's are also reflecting sunlight to the viewer when the sun is in the optic view angle of this particular LED. The reflection of the sun in a standard LED is higher than white paper in the sun and thus making correct signaling very hard. Only a high LED current and strong LED's can overcome the sun's reflection, but this is at higher initial investment and a shorter operational lifespan. Hood type LED masks have been reported which limit the viewing angle of the display. The disadvantage of these masks is that they are only effective if the angle of the sunlight is relatively large, i.e. 20 degrees or more with respective to the optical axis. More complex solutions have been reported also.
U.S. Pat. No. 8,249,375 B1 discloses an optical element for changeable traffic signs consisting of a light source, in particular, a light-emitting diode (LED), at least one converging lens and one diverging lens, which are arranged coaxially in a shared housing. The light exiting from the light source is captured as completely as possible by the converging lens, concentrated in a focal spot, which is preferably surrounded by a diaphragm and directed further onto the diverging lens which distributes it according to certain specifications. The refracting power of the diverging lens is dimensioned such that light exiting from it features a smaller angle of exit beta than a prescribed limit angle alpha. The distance between the converging lens and the diverging lens is dimensioned such that sunlight incident from the outside at an angle gamma greater than or equal to the limit angle alpha is completely blocked, either by the diaphragm or by absorption on the housing wall, so that no phantom light is generated.
A problem of the changeable traffic sign in U.S. Pat. No. 6,249,375 B1 is that the system is relative complex and thereby costly. For instance, an additional housing, with a pair of lenses and a diaphragm is required for each LED. Moreover, the relative placement of those parts has to be carefully taken care of also.